The invention relates to a configuration for spatially separating and/or joining at least two optical wavelength channels.
In special embodiments of such configurations, the grating device, which is used both, for separating and for joining the channels, has an optical grating and an optical free-radiating region which is provided between the grating and a point in space, which is assigned jointly to all the channels. The grating device also has a further optical free-radiating region which is provided between the grating and each point in space, which is assigned solely to one channel.
In one special embodiment of such a type, the grating includes a phased array, that is to say a plurality of strip-like optical waveguides, each of which has
in each case one end surface which faces the point in space which is assigned jointly to all the channels,
in each case one other end surface which faces the points in space each of which is assigned solely to in each case one channel, and
in each case one optical length between the one end surface and the other end surface, which varies from waveguide to waveguide.
If the special embodiment is operated as a demultiplexer in which the channels are spatially separated, the one end surfaces of the waveguides of the phased array form entry openings of the grating, and the other end surfaces of these waveguides form outlet openings of the grating. If this embodiment is operated as a multiplexer, in which the spatially separated channels are joined, the other end surfaces of the waveguides of the phased array form entry openings of the grating and the first end surfaces of these waveguides form outlet openings of the grating. The waveguides of the phased array act as an optical phase grating in any case.
Instead of a grating in the form of a phased array, other optical gratings, for example etched gratings, may also be used (see IEEE, Photonics Technology Lett., Vol. 8, No. 10, October 1996, pages 1340 to 1342).
The grating device of such a configuration governs a wavelength-dependent transmission function for each strip-like optical waveguide which is assigned solely to one channel or is assigned jointly to all the channels and has an end surface which faces the grating device and is provided at that point in space which is assigned solely to one channel or jointly to all the channels. At least to a first approximation, this transmission function is a Gaussian function (see IEEE, Photonics Technology Lett., Vol. 8, No. 10, October 1996, pages 1340 to 1342).
It would be preferable for the wavelength-dependent transmission function of such a waveguide to have a more rectangular profile in order that the insertion loss of this waveguide varies only insignificantly in a specific wavelength band when fluctuations occur in the ambient temperature and/or wavelength.
Various options have been described for flattening the inherent Gaussian-like transmission function of such a waveguide, that is to say to configure the transmission function such that it is more rectangular.
For example, it is known from the xe2x80x9cElectronics Lettersxe2x80x9d, 30, 1994, pages 300-301 for the waveguide which is assigned solely to one channel to be configured as a multimode waveguide rather than as a monomode waveguide, as usual, in order to flatten its transmission function.
It is also known for two slightly different phased arrays to overlap or be interleaved with one another so that, at the point in space in the configuration which is assigned solely to one channel, two spectrally deliberately shifted Gaussian-like transmission functions are superimposed to form a broader, flattened transmission function.
It is also known for a configuration to be configured such that, at the point in space in the configuration which is assigned jointly to all the channels, two overlapping Gaussian-like transmission functions are present. This can be achieved with a 3-dB beam splitter (see U.S. Pat. No. 5,412,744), with a so-called xe2x80x9cMultimode-Interferencexe2x80x9d coupler (see IEEE, Photonics Technology Lett., Vol. 8, No. 10, October 1996, pages 1340 to 1342) and/or with a so-called xe2x80x9chornxe2x80x9d structure (see Electronics Letters, 32, 1996, pages 1661-1662). The flattened transmission function produced at this point in space, in the form of the two overlapping Gaussian-like transmission functions, is mapped by the grating device onto each point in space in the configuration which is assigned solely to one channel.
In the three last-mentioned implementations, the critical process in the flattening is the formation of a convolution integral from an electrical field distribution in accordance with the overlapping Gaussian-like transmission functions, with the Gaussian mode of each waveguide configuration assigned solely to one channel.
It is known from the xe2x80x9cOptics Lettersxe2x80x9d, 20, 1995, pages 43-45 for the electrical field distribution to be varied at the other end surfaces of the waveguides of the phased array which form the outlet openings of the grating. The basis of this implementation is that the free-radiating region provided between these end surfaces and the separate points in space assigned solely to in each case one channel has a lens effect, and the electrical field distribution close to these end surfaces and the electrical field distribution close to these separate points in space are thus linked via a Fourier transformation. With a suitable choice of the cross-section of the waveguides of the phased array and an additional change to the optical length of these waveguides, it is possible to produce an electrical field distribution with, correspondingly, a sin(x)/x function close to the other end surfaces of these waveguides. This function is transformed by the Fourier transformation to a rectangular field distribution at a separate point in space.
It is accordingly an object of the invention to provide a configuration for spatially separating and/or spatially joining at least two optical wavelength channels which overcomes the above-mentioned disadvantages of the heretofore-known configurations of this general type and which can be easily constructed and in which, in a waveguide having an end surface provided at a point in space in the configuration, the wavelength-dependent transmission function of this waveguide can be adjusted easily and freely, without any spectral spreading of this function.
With the foregoing and other objects in view there is provided, in accordance with the invention, a configuration for spatially separating and/or spatially joining at least two optical wavelength channels, including:
an optical grating device defining a common spatial point, the optical wavelength channels having respective first optical powers commonly assigned to the common spatial point;
the optical grating device further defining separate spatial points assigned solely to respective ones of the optical wavelength channels, the optical wavelength channels having respective second optical powers respectively concentrated at the separate spatial points, and the optical grating device linking the respective first optical powers and the respective second optical powers;
strip-shaped optical waveguides optically coupled to the optical grating device, each of the strip-shaped optical waveguides being assigned solely to a respective one of the optical wavelength channels;
the strip-shaped optical waveguides having respective end faces respectively disposed at the separate spatial points assigned to the respective ones of the optical wavelength channels, the respective second optical powers being at least partially coupled to the strip-shaped optical waveguides;
the strip-shaped optical waveguides having respective wavelength-dependent transmission functions partly determined by the optical grating device;
an attenuator for providing an attenuation function for a wavelength-dependent attenuation of at least one of the respective wavelength-dependent transmission functions; and
the attenuator including an optical stop filter acting on an optical power transmitted in at least one of the strip-shaped optical waveguides assigned to the at least one of the respective wavelength-dependent transmission functions attenuated with the attenuation function, the optical stop filter having a wavelength-dependent filter curve forming the attenuation function.
This embodiment has the advantage that the grating device may remain unchanged and there is no need for any beam splitters, multimode interference couplers, horn structures or duplication of phased array structures.
With the objects of the invention in view there is also provided, a configuration for spatially separating and/or spatially joining at least two optical wavelength channels, including:
an optical grating device defining a common spatial point, the optical wavelength channels having respective first optical powers commonly assigned to the common spatial point;
the optical grating device further defining separate spatial points assigned solely to respective ones of the optical wavelength channels, the optical wavelength channels having respective second optical powers respectively concentrated at the separate spatial points, and the optical grating device linking the respective first optical powers and the respective second optical powers;
strip-shaped optical waveguides optically coupled to the optical grating device, each of the strip-shaped optical waveguides being assigned solely to a respective one of the optical wavelength channels;
the strip-shaped optical waveguides having respective end faces respectively disposed at the separate spatial points assigned to the respective ones of the optical wavelength channels, the respective second optical powers being at least partially coupled to the strip-shaped optical waveguides;
the strip-shaped optical waveguides having respective wavelength-dependent transmission functions partly determined by the optical grating device;
an attenuator for providing an attenuation function for a wavelength-dependent attenuation of at least one of the respective wavelength-dependent transmission functions; and
the attenuator including an attenuation device disposed in the grating device, the attenuation device attenuating an optical power of one of the optical waveguide channels in a position-resolved manner.
According to the above-defined solutions, the configuration according to the invention has only one additional device for producing an attenuation function for a wavelength-dependent attenuation of the transmission function of at least one waveguide having an end surface provided at a point in space in the configuration.
The wavelength-dependent attenuation function produced by the device is superimposed on a given transmission function of the waveguide and, through the use of this superimposition, is, converted to a different transmission function of this waveguide.
The wavelength-dependent attenuation function may be chosen freely so that, in principle, any desired transmission function of a waveguide can be produced from a given transmission function of this waveguide.
In particular, a flattened transmission function of the waveguide can be produced from a given transmission function of the waveguide which, for example like a Gaussian function, has a spike or a peak, by using an attenuation function which reduces the function values of the transmission function in the vicinity of the spike or peak to approximately constant function values. In this case, there is no need for any spectral spreading of the transmission function of the waveguide.
After this, the upper part of a given transmission function is cut off flat, thus providing the major advantage, in comparison with known configurations described above, that the solution according to the invention does not lead to any spreading of the transmission functions and the crosstalk characteristics of the configuration are thus not influenced by the flattening process.
Furthermore, the flattehing process according to the invention can advantageously be matched to a spectral width.
The stop filter can be produced in the relevant waveguide in a simple manner, for example in the form of a Bragg grating, for example with an excimer laser and phase masks, or holographically.
In consequence, it is advantageously possible to produce a stop filter, in accordance with customer requirements, for any waveguide channel with a different bandwidth and attenuation in a waveguide assigned to this channel wherein, advantageously, the flattening of the transmission function for each of these waveguides can be adjusted as required by the customer.
Irrespective of whether the configuration according to the invention is operated as a demultiplexer or a multiplexer, for each wavelength channel the stop filter can be configured in the waveguide which is assigned solely to this channel and/or in the waveguide which is assigned jointly to all the wavelength channels.
In accordance with a preferred feature of the invention, the attenuation device which is formed in the grating device for attenuating an optical power which is transmitted in the grating device can advantageously have an optical stop filter which extends transversely with respect to a propagation direction of the optical power and has a position-resolved transmission characteristic, which can be implemented in different ways.
In accordance with another feature of the invention, the grating device has an optical grating and defines a first optical free-radiating region and a second-optical free-radiating region. The first optical free-radiating region is provided between the common spatial point and the optical grating. The second optical free-radiating region is provided between the optical grating and the separate spatial points.
In accordance with yet another feature of the invention, the optical grating includes a plurality of further strip-shaped optical waveguides. The further strip-shaped optical waveguides have respective first end faces, respective second end faces, and respective optical lengths between the first and second end faces. The first end faces face the common spatial point, the second end faces respectively face the separate spatial points, and the optical lengths of the further strip-shaped optical waveguides differ from one another.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a configuration for a spatial separation and/or joining of optical wavelength channels, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.